RE: [PATCH] nfp: fix use-after-free in area_cache_get()

[Yinjun Zhang]

On Sat,  6 Aug 2022 22:30:43 +0800 Jialiang Wang wrote:
> area_cache_get() calls cpp->op->area_init() and uses cache->area by
>  nfp_cpp_area_priv(area), but in
>  nfp_cpp_area_release()->nfp_cpp_area_put()->__release_cpp_area() we
>  already freed the cache->area.

It frees only under condition that `area->kref` refcount is zero. But in normal path,
the refcount is incremented by calling `nfp_cpp_area_acquire` when `cache->id`
is not zero. So it's OK to release once when `cache->id` is not zero.

But it does have some problem in error path when `area_init` or `area_acquire`
fails, in which case `cache->id` is already set but refcount is not incremented as
expected. So we need set `cache->id` after everything is done.

So your current fix is not appropriate. Anyway thanks for bringing this to table,
I think you can resubmit a v2 to fix it?

> 
> To avoid the use-after-free, reallocate a piece of memory for the
>  cache->area by nfp_cpp_area_alloc().
> 
> Note: This vulnerability is triggerable by providing emulated device
>  equipped with specified configuration.
> 
> BUG: KASAN: use-after-free in nfp6000_area_init+0x74/0x1d0 [nfp]
> Write of size 4 at addr ffff888005b490a0 by task insmod/226
> Call Trace:
>   <TASK>
>   dump_stack_lvl+0x33/0x46
>   print_report.cold.12+0xb2/0x6b7
>   ? nfp6000_area_init+0x74/0x1d0 [nfp]
>   kasan_report+0xa5/0x120
>   ? nfp6000_area_init+0x74/0x1d0 [nfp]
>   nfp6000_area_init+0x74/0x1d0 [nfp]
>   area_cache_get.constprop.8+0x2da/0x360 [nfp]
> 
> Signed-off-by: Jialiang Wang <wangjialiang0806@xxxxxxx>
> ---
>  drivers/net/ethernet/netronome/nfp/nfpcore/nfp_cppcore.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_cppcore.c
> b/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_cppcore.c
> index 34c0d2ddf9ef..99091f24d2ba 100644
> --- a/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_cppcore.c
> +++ b/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_cppcore.c
> @@ -871,6 +871,10 @@ area_cache_get(struct nfp_cpp *cpp, u32 id,
>  		nfp_cpp_area_release(cache->area);
>  		cache->id = 0;
>  		cache->addr = 0;
> +		cache->area = nfp_cpp_area_alloc(cpp,
> +						 NFP_CPP_ID(7,
> +						 NFP_CPP_ACTION_RW, 0),
> +						 0, cache->size);
>  	}
> 
>  	/* Adjust the start address to be cache size aligned */
> --
> 2.17.1